Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as an alternative energy for replacing fossil fuels. In the solar cell, carriers (electrons and holes) generated by light irradiation on a photoelectric conversion section composed of a semiconductor junction or the like are extracted to an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit.
For example, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, collecting electrode(s) are provided on the transparent electrode layer(s). In this configuration, the transparent electrode layer can perform a function as a collecting electrode, and therefore in principle, it is not necessary to specially provide a collecting electrode. However, since a conductive oxide, such as indium tin oxide (ITO) or zinc oxide, forming the transparent electrode layer has a resistivity higher than that of metal, there is such a problem that the internal resistance of the solar cell increases. Thus, a collecting electrode (metal electrode as an auxiliary electrode) is provided on the surface of the transparent electrode layer to improve current extraction efficiency.
The solar cells are generally modularized by connecting a plurality of cells in series or in parallel via a wiring member. In modularization, the collecting electrode of the solar cell is electrically connected to the wiring member with an electroconductive resin adhesive or the like interposed therebetween.
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. On the other hand, a method of forming a collecting electrode by a plating method has also been developed. For example, a resist material layer having an opening matching the shape (e.g., comb shape) of a collecting electrode is formed on a transparent electrode layer of a photoelectric conversion section, and a metal layer is formed at the resist opening of the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a specific shape. Patent Document 1 discloses a method in which an insulating film composed of SiO2 is formed on a transparent electrode layer, an insulating layer is then patterned using a resist, and a metal layer is formed in the opening by electroplating.
The silver paste and the like contain a resin material, and therefore have a low electroconductivity, whereas the metal layer formed by a plating method has a high electroconductivity. Thus, when a collecting electrode is formed by a plating method, enhancement of efficiency of the solar cell (particularly, improvement of fill factor by reduction of resistance) can be expected. However, the resist material is expensive, and an under-layer forming step, a resist removing step, and the like are required for performing plating, so that steps for forming an electrode are complicated. Thus, the method of forming a collecting electrode in the opening of a resist by a plating method has the problem that material and process costs are increased to deteriorate practicability.
In view of the problem, a method has been proposed in which a collecting electrode of a solar cell is formed by a plating method without using a resist. For example, Patent Document 2 discloses a method in which an insulating layer of SiO2 or the like is provided on a transparent electrode, a groove extending through the insulating layer is then provided to expose the surface or side surface of a transparent electrode layer, and a metal collecting electrode is formed so as to be in conduction with the exposed part of the transparent electrode. Specifically, a method has been proposed in which a metal seed is formed on the exposed part of the transparent electrode layer by a light-induced plating method or the like, and a metal electrode is formed by electroplating with the metal seed as an origination point. This method is more advantageous in terms of material and process costs because it is not necessary to use a resist. By providing a metal seed having low resistance, contact resistance between the transparent electrode layer and the collecting electrode can be reduced.
Patent Document 3 proposes a method in which a discontinuous insulating layer having an opening is formed on an electroconductive seed, and a metal electrode is formed via the opening of the insulating layer by electroplating.